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A miniature high-temperature fixed point for self-validating type C thermocouples.

Ongrai, O; Pearce, J V; Machin, G; Sweeney, S J (2011) A miniature high-temperature fixed point for self-validating type C thermocouples. Meas. Sci. Technol., 22 (10). 105103

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Reliable high temperature (>1500 °C) measurement is crucial for a wide range of industrial processes, as well as specialised applications in for e.g. aerospace. The most common type of sensor used for high temperature measurement is the thermocouple. At and above these temperatures, tungsten-rhenium (W-Re) thermocouples are the most commonly used due to their utility up to 2300 °C. However the achievable accuracy of W-Re thermocouples is seriously limited by the effects of their inhomogeneity, drift, and hysteresis. Furthermore, due to their embrittlement at high temperature, the removal of these thermocouples from environments such as nuclear power plants or materials processing furnaces for recalibration is generally not possible or even desirable due to large inhomogeneity effects. Ideally, these thermocouples require some mechanism to monitor their drift in-situ.
In this study, we describe how a miniature Co-C eutectic fixed point cell in a graphite crucible was fabricated to evaluate the stability of type C (W5%Re/W26%Re) thermocouples by means of in-situ calibration. Type C thermocouples with wire diameter 0.25 mm (hafnia insulation) and with an outer molybdenum sheath of diameter 1.5 mm and length 375 mm was studied. The miniature Co-C eutectic cell was constructed in a graphite crucible of outside diameter 10 mm, length 18.5 mm with internal volume 0.36 cm3. The thermal influences on the performance of such a small fixed-point cell, and the repeatability of the thermocouple/fixed point combination were investigated and are reported here.

Item Type: Article
Keywords: tungsten-rhenium thermocouple, in-situ calibration, self-validating thermocouple, Co-C eutectic cell, fixed point
Subjects: Engineering Measurements
Engineering Measurements > Thermal
Identification number/DOI: 10.1088/0957-0233/22/10/105103
Last Modified: 02 Feb 2018 13:14
URI: http://eprintspublications.npl.co.uk/id/eprint/5275

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